Cross-Laminated Timber Opens New Possibilities for Wood Construction

Wood is a desirable construction material for many reasons including its low embodied energy. But, until recently, it has not been possible to build tall wooden structures because of the relative weakness of conventional wood stud construction methods. This is starting to change as a new method of fabricating wood panels, called cross-laminated timber, or CLT, is making “massive wood” construction a possibility for mid-rise construction, as well as for other construction uses.

As a building material, cross-laminated timber (CLT) is a rather new development. The first work with CLT was done in Switzerland and Austria in the early to mid 1990s.

Because the panels are lighter than masonry and hollow-core concrete plank, and since the panels can be lifted with lighter equipment, construction can be faster and thereby less expensive. It also requires a smaller crew to install and connect the panels at the job site, which allows for rapid and efficient construction.

In one example, a 9-story building saved an estimated 22 weeks of construction time by using CLT instead of concrete. The panels are prefabricated in a shop, so the benefit of controlled conditions is present, as with structural-insulated panels (SIP) and other pre-fab materials.

CLT is used for both walls and floors. The panels are made as alternating boards are laid perpendicular to one another (the same thing is done with plywood) for greater strength. Since the CLT panels are predominantly solid wood, there is far less glue in them than an equivalent volume of plywood. CLT panels have been tested for VOC off gassing from the glues, and are far below regulatory limits in that regard.

Since they are prefabricated, CLT panels can be installed with tighter tolerances and provide more plumb and true construction. Solid wood is not a great insulator, but wood is better than CMU or steel, and the building can still be insulated on the exterior or interior.

Using a massive wood structure — from a carbon cycle perspective — not only produces far lower emissions during the manufacture of the material, but the wood also sequesters carbon for its lifespan.

Although one might think it would be a fire hazard, massive wood construction is actually fairly fire resistant. For decades, building codes have recognized this for buildings constructed with large solid-sawn post and beam construction. Wood will char on the surface, but it takes a very long time for it to burn through. CLT is the same.

At present, the only manufacturers of CLT panels are in Europe, but two plants in Canada and one in the United States are currently under construction. There are a number of examples of mid-rise apartment buildings constructed with CLT in Europe. The first North American non-residential CLT structure was a bell tower completed in 2010 in North Carolina.

[Ed. Note: In my backyard, the University of Utah’s Integrated Technology in Architecture Center is working on an ICLT — interlocking cross laminated timber — system with no fasteners or adhesives. It’s in development and testing with a 3-5 horizon for commercial availability. — Preston]

Using CLT won’t make a fireproof building (any more than any other building material), and codes and fire suppression systems will still pertain. But building with CLT allows for a building that will offer fire resistance and durability more along the lines of other materials, allowing wood to be used in mid-rise buildings which previously could not be done.

As pliable as wood is and of woods use as a favored building material there should still be as many other alternatives for the building industry than wood from a green perspective such as the use of polymer ties and beams enhanced to be lighter with a secret bonding process produced with the same if not better tolerances as wood and longer cheaper lifespans for the building industry try http://www.novenco.com NCL Novenco Polymer Industries Val Caron Industrial Park Sudbury ONT CAN .

The featured photo shows the new University of British Columbia earth sciences building being constructed with Structurlam’s cross laminated timber. These are indeed building panels as opposed to manufactured structural beams. Structurlam Products Ltd is currently one of the few manufacturers of this renewable product. We’d be happy to address any questions about cross laminated timber: http://www.structurlam.com/contact/

CLT technology (its finally arriving in the US!) offers some significant advantages for construction including the ability to integrate BIM/CAD with CNC machinery for precision-cutting of panels, openings and so on. This avoids job-site waste and poorly-performing joins and junctions between elements that are usually associated with leaving “the details” to the builder or subcontractors armed with caulking guns and nailers.

Roof decks spanning upwards of 25ft between supports (depending on panel thickness and snow/wind loadings of course) also means that labor-intensive expensive lighter-gauge framing using closely-spaced joists can also be eliminated.

Its not the be-all and end-all of construction materials but as a substitute for ridiculously-poorly-performing stick-framing and stucco of the kind we see far too much of in the desert southwest, CLT offers up opportunities for smarter, more sustainable solutions.

I still maintain that high-rise buildings could use more wood materials than being used now. The main structural support would still be the elevator cores’ shear walls w/ pre-cast reinf. conc. or steel beams & girders for structural frames but all floors, interior & exterior walls, ceilings, etc. could be wood or CLTs in some areas. With regards to fire safety, fire-detection & suppression systems will still be in-place as in a conventional steel / concrete structure. This would significantly reduce concrete & steel use.. The only constraint is in some countries wood is a scarce resource that cost will skyrocket if more wood is used…